The present application claims priority to Japanese Application No. P2000-121339 filed Apr. 21, 2000, P2000-145601 filed May 17, 2000 and P2001-060394 filed Mar. 5, 2001, which applications are incorporated herein by reference to the extent permitted by law.
The present invention relates to beta-type nickel oxyhydroxide and a method of producing thereof, and yet, also relates to a positive electrode active material composed of beta nickel oxyhydroxide. Further, the present invention relates to a nickel-zinc battery incorporating a positive electrode comprising beta nickel oxyhydroxide as a positive electrode active material and a negative electrode comprising zinc as a negative electrode active material.
In recent years, compact-size portable electronic apparatuses, especially portable game players, digital cameras and digital video-camera recorders, or the like, have been propagated very significantly. It is expected that these compact-size portable electronic apparatus will be propagated furthermore from now on, and thus, demand for compact-size battery serving as a power-supply source for these compact-size portable electronic apparatuses will also be promoted quickly. Generally, any of those compact-size portable electronic apparatuses utilizes a high operating voltage and requires a large amount of current, and thus, a usable power source must be distinguished in discharge characteristic under heavy load.
Of those batteries satisfying the above requirements, such a alkaline-manganese battery has already been propagated most widely, which comprises manganese dioxide for composing the positive electrode and zinc for the negative electrode, and yet, it also comprises highly concentrated alkaline aqueous solution for composing electrolytic solution. Inasmuch as manganese dioxide and zinc are respectively inexpensive, and yet, because of high energy density per weight, not only for the power-supply source of compact-size portable electronic apparatuses, but the alkaline-manganese battery is also utilized extensively.
Considering further utility for compact-size portable electronic apparatuses and in order to further improve discharge characteristic under heavy load, a wide variety of improvements have been achieved in a range from battery material to the composition of battery itself. However, in the above alkaline-manganese battery, inasmuch as a positive electrode active material comprising manganese dioxide performs discharge based on homogeneous solid-phase chemical reaction, as a result of discharge, voltage gradually lowers whereby drawing such a discharge curve of downward-sloping. Because of this, in such a compact-size portable electronic apparatus requiring a high voltage and a large amount of current, basically, discharge performance of the alkaline-manganese battery can hardly suffice practical need, and yet, despite of a variety of improvements thus far effected, duration of actually operable capacity of such a compact-size portable electronic apparatus has thus been extended by a negligible extent. Further, any of the modern compact-size portable electronic apparatuses is apt to perform own operation with a relatively higher voltage and a greater amount of current in the initial stage of distribution in the market. To deal with this tendency, it is imperative that such a battery compatible with a newer model of any of compact-size portable electronic apparatuses and capable of preserving distinguished durability to heavy load be provided as essential requirements.
To suffice the above requirements, a nickel-zinc battery has thus been proposed. The nickel-zinc battery comprises its positive electrode comprising nickel oxyhydroxide and its negative electrode comprising zinc, which contains such an operating durability to heavy load respectively being higher than those of the above alkaline-manganese battery. On the other hand, nickel oxyhydroxide as a positive electrode active material easily generates oxygen and a large amount of self-discharge as problems to solve. As a method for solving these problems, for example, the Japanese Laid-Open Patent Publication No. HEISEI-10-214621 (1998) proposes such a nickel-zinc battery having an xe2x80x9cinside-outxe2x80x9d type structure with a less amount of self-discharge by utilizing gamma nickel oxyhydroxide (xcex3-NiOOH) for composing a positive electrode active material.
Such a battery utilizing the above-cited gamma nickel oxyhydroxide has a small amount of self-discharge, and has higher operating potential than that of an alkaline manganese battery. However, it is a problem that such a battery cannot have large discharge capacity because the above gamma-type nickel oxyhydroxide has relatively low density.
The object of the present invention is to provide such a nickel-zinc battery having such a discharge voltage higher than that of an alkaline-manganese battery and distinguished in the large-current discharge characteristic.
The present invention introduces such a positive electrode active material comprising beta-type nickel oxyhydroxide consisting of substantially spherical particles. Preferably, mean particle size of the beta nickel oxyhydroxide is within a range from 19 xcexcm to a maximum of 40 xcexcm. Preferably, bulk density of the beta nickel oxyhydroxide is within a range from 1.6 g/cm3 to a maximum of 2.2 g/cm3. Preferably, tap density is within a range from 2.2 g/cm3 to a maximum of 2.7 g/cm3. Preferably, specific surface area of beta nickel oxyhydroxide based on BET method is within a range from 3 m2/g to a maximum of 50 m2/g.
The above-referred beta nickel oxyhydroxide for composing the positive electrode active material used for implementing the present invention is previously treated with alkaline aqueous solution and contains alkaline cation between layers of the beta nickel oxyhydroxide.
Further, the nickel-zinc battery proposed by the present invention utilizes the above-referred beta nickel oxyhydroxide for composing positive electrode active material. The positive electrode at least contains beta nickel oxyhydroxide and graphite powder, where the weight ratio of graphite powder against a total weight of the positive electrode is defined within a range from 4% to a maximum of 8%.
According to the present invention, it is possible to secure such nickel oxyhydroxide with least self-discharge, and yet, such a nickel-zinc battery using said nickel oxyhydroxide for composing positive electrode active material as the one embodied by the invention generates such an operating voltage and distinguished durability to heavy load respectively being higher than those of conventional alkaline-manganese batteries.